1. Field of the Invention
The present invention relates to a front derailleur for use in a bicycle, and more particularly to a front derailleur of the above-noted type having a link mechanism with a plurality of links including a movable first link and a second link fixedly secured to a bicycle frame; a chain guide operatively connected to the movable first link; urging means for urging the chain guide; and a control unit operatively connected to the link mechanism for transmitting a control force to the same. With an input of control force from the control unit, the link mechanism is pivoted to move the chain guide against the urging force of the urging means thereby selectively engaging the chain with a first sprocket positioned at an urging side end of the urging means, at least one middle sprocket and a second sprocket opposed to the first sprocket across the middle sprocket.
2. Description of the Related Arts
A conventional bicycle front derailleur, as illustrated in FIG. 6, employes a parallel quadruple link mechanism of the equilateral type, with opposed link pairs respectively having same inter-pivot distances L1, L2 and L3, L4, for fixedly maintaining the posture of the chain guide (e.g. a Japanese published untility model No. 57-39271). For effecting a change speed operation with this derailleur, a control unit 29 for the link mechanism 20 is pulled via an unillustrated cable against the urging force of urging means 40 attached to the link mechanism thereby moving the chain guide 30 in a direction opposed to the urging direction of the urging means 40 (this direction will be referred to as `reverse urging side` hereinafter). Alternately, the cable is released by a predetermined amount to move, in cooperation with the urging force of the urging means, the chain guide along the urging direction (this direction will be referred to as `forward urging side` hereinafter).
With the above construction, as known from the Hook's law, the advancement of chain guide 30 towards the forward urging side associates a decrease in deformation amount of the urging force and consequently a decrease in the urging force thereof to be used for further moving the guide in this direction. Accordingly, when the cable is released for another speed change, this change speed operation from the middle sprocket 53 to the first sprocket 51 positioned in the forward urging side cannot be effected smoothly as compared with the change speed operation from the second sprocket 52 positioned in the reverse urging side to the middle sprocket 53.
Furthermore, if the urging means 40 has a strong urging force, as the chain guide moves towards the reverse urging side, there occurs increasing resistance in the urging force against the pulling force F of the cable to be used for moving the guide towards the forward urging side. Therefore, a greater operation force is necessary for a change speed with moving the chain guide from the middle sprocket 53 to the second sprocket 52 than that required for a change speed with moving the same from the first sprocket 51 to the middle sprocket 53. As the result, the change speed operation from the middle sprocket 53 to the second sprocket 52 cannot be effected smoothly.
With view to the above-described state of the art, the primary object of the present invention is to provide an improved bicycle front derailleur capable of reliably effecting a change speed operation regardless of the sprocket-position of the chain guide with the aid of a sufficient urging force and achieving minimized difference in operation forces required for various change speeds.